212 BONY FISHES vn. 13- 



13. Receptors for life in the water 



The features of the environment that are relevant for life are very 

 different in air and water. Man is so well used to the air that it is not 

 easy to appreciate fully the conditions underwater, where changes of 

 illumination, though obviously important, provide less detailed evid- 

 ence of the sequence of distant events than they do in air. We can 

 say that light carries less information for a fish; to put it in another 

 way, fewer distinct choices between alternative behaviour pathways 

 are made on the basis of visual clues by a fish than by a man. 



On the other hand, the water around the fish provides mechanical 

 stimuli both at low and high frequency that are more closely related 

 to distant events than is generally true in air. Both hearing and touch 

 are of great importance in the water and the lateral line system pro- 

 vides a system of 'distant touch' that is perhaps wholly outside our 

 experience. Localization of distant objects by such a sense, perhaps 

 assisted by echo-location by water movements, provides the fish with 

 many relevant clues. It is interesting that these receptors are connected 

 with a very large cerebellar system, perhaps concerned with measur- 

 ing time differences. 



Chemical changes in the water also provide much information and 

 both taste and smell are well developed. That smell is analysed by a 

 distinct system in the forebrain, not directly related to the cerebellar 

 system, is one of the fundamental principles of control of vertebrate 

 behaviour. Distant chemical changes provide the first clue to the 

 presence of food, a mate, or an enemy, whereas the detailed finding 

 of these involves eyes, ears, touch, and an accurate timing system. 

 There thus arises the distinction between the systems for initiation of 

 action in the forebrain ('emotive') and for its fulfilment (executive) 

 by centres farther back. 



14. Eyes 



An animal provided with suitable receptors can obtain much 

 information about the environment from the changes in illumination. 

 Control of the whole physiology to follow the rhythm of day and night 

 may have been the original reason for the development of photo- 

 sensitivity in the diencephalon (see p. 105). At the stage of evolution 

 reached by teleosts information is gained from the fact that light 

 varies in frequency (colour) and intensity (brightness) and that it is 

 reflected from many substances, revealing their movement and shape. 

 The greatest sensitivity of the fish eye is in the yellow-green, which 



